Neuronal development involves a carefully choreographed combination of cell fate assignment, cell proliferation and cell migration. The coordination of these processes results in the stereotyped regions of the brain in the central nervous system and the patterned set of ganglia in the peripheral nervous system. Faced with the complexity of studying such interleaved processes and the dramatic advances in molecular biology, research in the peripheral nervous system has been assisted by high resolution imaging studies within the developing organism. For example, laser scanning confocal microscopy of labeled clones of cells within the neural crest has allowed the cell lineages to be directly assessed, and the patterned migration to be directly followed; when combined with molecular approaches, this has resulted in new insights to the molecular and cellular mechanisms of peripheral nervous system development. The goal of the proposed experiments is to perform such a coordinated analysis in the central nervous system of a vertebrate, using the zebrafish cerebellum. The cerebellum has been one of the most intensely studied regions of the central nervous system. The defined laminar structure of the cerebellum, which is well-conserved between species, emerges during embryogenesis from the long distance migration of neuronal precursors along specific migratory pathways. The cerebellum form in a position that permits the precursor cells and their complete migration pathway to be imaged with subcelllular resolution within the living zebrafish embryo. The proposed studies will build upon our earlier imaging experiments of cells within the forming zebrafish cerebellum to assess the molecular basis of neuronal guidance. Defined perturbations of defined signaling pathways will be used to explore their role(s) during each phase of the precursor cell migration. Because the structure of the cerebellum is well-conserved between species, these studies on lower vertebrates will offer important insights into the higher vertebrate and human neuronal development.